Leflunomide, i.e. 5-methyl-N-(4-trifluoromethylphenyl)isossazole-4-carboxamide), is a molecule described in EP-B-0 013 376 as an antinflammatory, analgesic and antipyretic agent in the treatment of rheumatoid arthritis, as it also has immunosuppressive activity. Ann. Rep. Med. Chem., 18, 177 (1983) states that its activity is actually exerted by a metabolite, A-77-1726 N-(4-trifluoromethylphenyl)-1-cyano-2-ketopropyl-carboxamide!, in particular by the .beta.-ketonitrile system (which is liable to keto-enol tautomerism) contained therein. In fact, when replacing the cyano group with an ester group, the activity dramatically decreases or even disappears (J. Chem. Soc. Perkin Trans, 1, 2203 (1992)!.